International Patent Application Nos. PCT/AU84/00192 and PCT/AU86/00049 describe methods for backwashing elastic microporous hollow fibre filters The filters disclosed in these applications consist of a bundle of polymeric (such as polypropylene) fibres contained within a housing having a feedstock inlet thereto and a concentrate outlet therefrom. The feedstock is applied to the outside of the fibres and some of the liquid contained in the feedstock passes through the walls of the fibres and is drawn off from the fibre lumens as filtrate.
The fibres are cast in resin at both ends of the shell or housing with the ends of the lumens open to constitute a tube-in-shell configuration. Although not described in our above mentioned specifications, the fibres may be cast into one end of the housing with the other ends of fibres free but with the free ends of the lumens sealed to constitute a candle-in-shell configuration.
During the filtering operation, which may be either to recover clarified liquid or to recover concentrated solids, solids contained in the feedstock either pass out of the shell with the remainder of the feedstock carrier stream, or are retained on or in the fibres. These retained solids cause fouling and blockage of the filter. Industrial practice with the more common tube-in-shell microfilters for many years was commonly to apply the feedstock to the inner surface of the fibres by forcing flow through the fibre lumens at such a rate that turbulence scoured the walls of the fibres, retarding blockage by solid material.
In the above mentioned specifications, the feedstock is applied to the outer surface of the fibres, with a penalty of low feedstock flow velocity and consequent low turbulence resulting in a rapid rate of blockage of the pores of the fibres. This is overcome by the application of a two-stage backwashing cycle.
In the first stage a liquid backwash is applied to the lumens of the fibres such that the liquid passes through the porous walls of the fibres and sweeps retained solids out of substantially all of the pores in the walls of the fibres. In the second stage, a gaseous backwash is applied to the lumens of the fibres such that the gas passes through the larger pores in the walls of the fibres, stretching them and dislodging retained blocking solids.
International Patent Application No. PCT/AU86/00049 discloses a method of applying pressure such that the gaseous backwash is applied evenly over the inner surface of the hollow fibres. In this method, the volume of liquid backwash is that volume of liquid trapped in the pores of the walls of the fibres. When the backwash stage is begun, low pressure gas is applied to clear the fibre lumens of liquid, and then high pressure gas is applied so as to exceed the bubble point of the fibres and force gas through the larger pores in the fibre walls.
The application of the two-stage backwashing regime discussed above restores filtrate flux to a high value that is, however, not as high as the initial value At each stage this slight diminution of flux reduces the filtration capacity of the fibres. Eventually chemical cleaning is required. This is expensive and time consuming
Another method of cleaning the fibres is known as reverse flow and is reviewed in "Ultrafiltration Membranes and Application", Edited by A.R. Cooper, a record of a Symposium of the American Chemical Society, Sept. 11-13, 1979, Pages 109 to 127, "Advances in Hollow Fibre Ultrafiltration Technology", by B.R. Breslau.
In the Breslau method the feed is applied to the lumens of the fibres at high velocity so that there is a large pressure drop down the length of the fibres By closing off the filtrate flow at the distal end of the shell, the filtrate pressure climbs within the shell and forces filtrate backwards through the fibre walls in the distal end of the fibre bundle. The direction of flow of feedstock is then reversed and the process repeated so as to force filtrate backwards through the fibre walls in the proximal end of the fibre bundle Filtrate is produced in one end of the shell and used to backwash the fibres at the other end of the shell.
A distinction is made between the term "reversed flow (filtering)" as used by Breslau and the description of reversing the direction of flow while no filtering is occurring as hereinafter described.
The prior art also contains a number of references to filter systems which utilize pressure variations arising,
For example, German specification No. 2,833,994 discloses a filtration process in which two fluid streams flow countercurrent to each other on either side of a filter medium. The flow of filtrate is subjected to a series of reductions of the flow cross section. These reductions with the associated acceleration in velocity induce a region of low pressure below the membrane, causing a flow of fluid through the membrane
Netherlands specification No. 7,604,657 discloses a method for cleaning tubular membranes in which gas is dissolved in a liquid under pressure. The liquid is fed past the membrane and the pressure is reduced so that gas is released as small bubbles which lift solids from the membrane and carry them away.
Similarly, the feeding of gas - liquid mixture to the surface of the membrane is taught by Japanese specifications 61-129094 and 56-024006.
The cleaning of dead-end fibres dangling in a pot by ga cleaning causing writhing of the fibres is disclosed in British patent No. 2,120,952. Japanese specification 60-137404 teaches the installation of special equipment to vibrate dead-end fibres hanging in a pot during backwash and Russian specification No. 715,105 discloses air pulsing of wash water applied to a granulated filter.
Japanese specification No. 53-042186 teaches the periodic reversal of direction of flow of feed liquid in a membrane plate separator. Japanese specification No. 61-101209 discloses a method of applying a vacuum to eliminate air from the pores of a hydrophobic membrane.
Japanese specification No. 47-021748 discloses the reversal of application of air pressure. First air pressure drives liquid through the membrane. When backwashing with filtrate is required, the air pressure is applied to the filtrate. When a flow meter indicates sufficient washing, the air pressure is again applied to the feed side to restart the filtration.
The article "Anti-fouling Techniques in Cross-flow Microfiltration" by Milisic & Bersillon, Filtration &
Separation, November/December 1986, pp 347-349, teaches pulsing the feedstream as it is applied during normal filtration.
Banks of fibres in shell filter cartridges are frequently arranged in parallel. When one shell develops a blockage, flow bypasses this fibre bundle, the velocity slows, and the blockage becomes self-increasing through the system.
The need to optimize the frequency of cleaning cycles to maximize filtrate flow is discussed in International Patent Application No. PCT/AU84/00192.
For the procedure described to be successful, the fibres must be elastic. For practical considerations of each of manufacture and resistance to acid cleaning that must be applied eventually, and for strength, the fibres are generally chosen to be a thermoplastic such a polypropylene. Such thermoplastics are fundamentally hydrophobic and must be wetted before they can be used to filter aqueous feedstock streams.
The application of backwashing gas as described above has the undesired effect of partially drying the fibres. Small bubbles of gas are retained in the pores in the walls of the fibres where they effectively block filtration. The filtrate flux is initially high at the start of filtration, but rapidly drops as the fibres foul with solids. The application of the two stage backwashing regime restores the filtrate flux to a high value that is however, not as high as the initial value. At each stage this slight diminution of flux reduces the filtration capacity of the fibres. Eventually chemical cleaning and/or rewetting is required which is expensive and time consuming.
International Patent Application No. PCT/US83/02004 discloses the pressurized initial wetting of fibres in relation to cartridge units that are intended for a special use such as with blood, and which can be prewetted before shipment. However, in industrial situations, cartridges may be used for many applications that are not specified at the time of manufacture of the cartridge. For applications such as food use, the presence of extraneous wetting agents such as surfactants must be avoided and there is a need to wet the fibres with the liquid to be filtered. In these cases it is impractical to wet the fibres during manufacture. They must be wetted in place, immediately prior to use. The procedure described in our International Patent Application No. PCT/AU86/00049 utilizes a flow of feedstock to wash away the dislodged solids. However, it is sometimes necessary that the solid material be recovered in a dryer state than is the case with the processes described in our above International Patent Applications. This is particularly useful where solids recovery and dewatering are important.
According to the invention there is provided a method of operating a filter having elastic, porous, hollow fibres within a shell or housing comprising the steps of:
(i) introducing a liquid suspension feedstock into the shell or housing and directing said feedstock to the outer surface of the fibres whereby:
(a) some of said feedstock passes through the walls of the fibres to be drawn from the fibre lumens as a filtrate or permeate, PA1 (b) some of the solids in said feedstock are retained on or in the pores of the fibres, with the non-retained solids being discharged from the shell or housing with the remainder of said feedstock, PA1 (a) introducing a pressurized gas into the fibre lumens which passes through the walls of the fibres to dislodge the retained solids, and, PA1 (b) varying the pressure within the shell whilst the gas is being introduced into the lumens. PA1 (a) some of said feedstock passes through the walls of the fibres to be drawn from the fibre lumens as a permeate, PA1 (b) some of the solids in said feedstock are retained on or in the pores of the fibres with the non-retained solids being removed from the shell with the remainder of said feedstock, PA1 (a) introducing a pressurized liquid through the fibre lumens which passes through the walls of the fibres to wash out at least some of the retained solids and then, PA1 (b) introducing through the fibre lumens a pressurized gas which passes through the walls of the fibres and stretches elastically at least some of the pores to dislodge any solids retained in those pores and which washes the external walls of the fibres, the gas being applied at a pressure which is sufficient to overcome the resistance to gas flow of the surface tension of the continuous phase of the filtrate within the pores of the membranes, and, PA1 (c) varying the pressure within the shell whilst the pressurized gas is being introduced into the lumens.
(ii) periodically cleaning away the retained solids by:
According to another aspect of the invention, there is provided a method of operating a filter having a plurality of elastic, microporous hollow fibres with a shell or housing comprising the steps of:
(i) introducing a liquid suspension feedstock into the shell or housing and applying said feedstock to the outer surface of the fibres whereby:
The pressure within the shell may be varied during cleaning in a number of ways such as by :increasing the pressure within the shell above the normal gaseous cleaning pressure and then returning the pressure to the normal gaseous cleaning pressure or by decreasing the pressure within the shell below the normal gaseous cleaning pressure and then returning the pressure to the normal gaseous cleaning pressure.
The pressure within the shell may be increased by terminating the outflow of feed and then returned to normal gaseous cleaning pressure by recommencing flow of feed in either the same or the reverse direction.
The pressure within the shell may be decreased by terminating the inflow of feed and the return to normal gaseous cleaning pressure can be achieved by resuming inflow of feed in the same or the reverse direction.
The methods of the invention may be modified by terminating the inflow of feed before commencing the gaseous backwash step to effect a dry backwash. The feed flow may also be replaced by a high or low pressure gas through the inlet to the shell so as to assist the discharge of the retained solids.
All the above variations in the mode of operating the filter during the cleaning cycle may be repeated a number of times during gaseous cleaning.
In one form of the invention, the shell is pressurized by terminating feed flow before the pressure variation step and the pressure is released by recommencing feed outflow prior to the application of the pressure variation step.
In a modification of this form of the invention, the pressure is released at both the feed and recirculation ends of the shell.
The methods of the invention may also be modified by including a step of pressurizing the fibres after the completion of the backwash and then releasing that pressure to remove trapped air from the pores of the fibres. The step of pressurizing the fibres may be carried out by terminating the feed inflow and feed outflow and the pressure may be released by recommencing feed inflow with or without recommencement of feed outflow. The pressurization of the fibres is carried out whilst lumen flow is blocked preferably in a pulsing fashion.
To carry out the pressurization, after the backwash cycle has been completed, the feedstock and filtrate flow are blocked. A hydraulic pressure preferably from a piston of pressurized gas is applied to either the filtrate side of the fibres or the feedstock side of the fibres, or both. Thus pressure is applied to the fibres and the compressible gas contained in the pores of the fibres is reduced in volume or dissolved in the liquid in the fibres due to its greater solubility under pressure. On resumption of feed flow the gas is expelled In some circumstances it may be preferable to drain the fibre lumens before commencement of the gaseous backwash step. Furthermore, it may be advantageous to drain the shell before commencement of backwash.
According to another aspect of the invention, the introduction of the pressurized gas for cleaning includes the steps of:
(a) initially applying the gas at a pressure below the bubble point of the walls of the fibres so as to displace any liquid from the fibre lumens,
(b) terminating feed inflow and outflow,
(c) increasing the pressure of the gas above the bubble point of the walls of the fibres, and,
(d) recommencing feed inflow and outflow to allow the trapped gas to escape substantially uniformly through the fibre walls.
Preferably, the introduction of the pressurized gas during the dry backwash includes the steps of:
(a) introducing another gas into the shell side of the fibres at a pressure substantially the same as the lumen cleaning gas,
(b) terminating the flow of the shell side gas, opening the shell inlet and/or shell outlet to release the gas pressure on the shell side of the fibres and to allow the lumen gas to escape substantially uniformly through the fibre walls.
The filter may be operated in a cross flow mode or in a dead-end filtering mode with no outflow of feed and solids from the shell, during the dead-end filtration mode.
In yet another embodiment of the invention, the backwashing cleaning step is enhanced by discharging through both the shell inlet and outlet and feeding through an additional line connected to the shell between the shell inlet and outlet.
The invention also includes apparatus for carrying out the methods described above.